The present invention relates to a method for manufacturing a lithium ion secondary battery comprising a separator containing electrolyte, a positive electrode and a negative electrode facing each other with the separator therebetween. More specifically, the present invention relates to a method for manufacturing a battery which can take optional form such as a flat type and is compatible with adherence and electric connection among a separator, a positive electrode and a negative electrode.
There is a great demand for miniaturization and weight reduction of portable electric appliances, and their realization requires improvements in battery performance. In recent years, various batteries have been developed and improved for improvements in the battery performance. The expected improvements in the battery performance are higher voltage, higher energy density, higher resistance to load, formability for optional form, safety and the like. A lithium ion secondary battery has been improved enthusiastically even in a recent year, since it has the highest voltage in a single battery in all kinds of batteries present and can realize higher energy density and higher resistance to loads.
A lithium ion secondary battery contains a positive electrode, a negative electrode, and an ion conducting layer sandwiched between these electrodes as a main component. In the practical lithium ion secondary battery, the positive electrode is generally made by mixing an active material powder such as a lithium-cobalt oxide composite with an electronic conductive powder and a resin to bond these powders, and applying the mixture onto an aluminum current collector to form it into a plate. And the negative electrode is generally made by mixing a carbon-based active material powder with a binding resin, and applying the mixture onto a copper current collector to form it into a plate. The ion conducting layer is generally made of a porous film such as polyethylene or polypropylene impregnated with a non-aqueous solution containing lithium ions.
For example, FIG. 9 shows a cross sectional view of the structure of a conventional cylindrical lithium ion secondary battery disclosed in Japanese Unexamined Patent Publication No. 83608/1996.In FIG. 9, 1 is a solid casing made of stainless or the like which also serves as a negative electrode terminal, and 2 is an electrode assembly stored in the solid casing 1. The electrode assembly 2 comprises a positive electrode 3, a separator 4, and a negative electrode 5, which are coiled together. The electrode assembly 2 must give pressure from outside to its electric surface in order to maintain the electric connection among the positive electrode 3, the separator 4 and the negative electrode 5. To maintain all the contacts inside the surface, the electrode assembly 2 is stored in a strong metal casing. In the case of a square-shaped battery, strip-like electrode assemblies tied in a bundle are stored in a square-shaped casing and pressed with an external pressure.
As described above, in a commercially available lithium ion secondary battery, a strong solid casing made of a metal or the like is used as a means to adhere the positive electrode to the negative electrode. Without the solid casing, a distance between the electrode surfaces becomes far each other, failing to maintain the electric connection between the electrodes via the ion conducting layer (a separator impregnated with a non-aqueous electrolyte), thereby deteriorating the battery properties. Since a volume and a weight of the solid casing is large in the whole battery, it decreases energy density in a battery unit volume or a unit weight, and also limits the possible form of the battery due to the stiffness of the solid casing. Thus, it is difficult to obtain a desired form.
Under those circumstances, in order to realize a reduction in weight and thickness, a lithium ion secondary battery which does not need a strong solid casing have been developed. The key point to the development of the battery which does not require a strong solid casing is to successfully maintain the electric connection between the positive electrode, the negative electrode and the ion conducting layer sandwiched therebetween without applying an external pressure. One proposed method of joining the electrodes with the ion conducting layer without external force is to use a resin or the like.
For example, Japanese Unexamined Patent Publication No. 159802/1993 discloses a method for combining an ion conductive solid electrolyte layer, a positive electrode, and a negative electrode by heating using a thermoplastic resin binder. According to the method, these electrodes are joined with each other by uniting the positive electrode, the negative electrode and the solid electrolyte layer, so that the electric connection between these electrodes and the solid electrolyte is maintained without applying any external pressure, which makes it possible to function as a battery.
The conventional lithium ion secondary battery using a strong solid casing to secure adherence and electric connection between the positive and negative electrodes and the separator has a drawback that the solid casing, which is not included in the electric generator, makes up a large proportion of the entire battery in weight and volume, becoming a disadvantage to manufacture a high energy density battery. It seems possible to use an ion conducting adhesive resin to adhere a positive electrode, a negative electrode and the separator, but there arises a problem that when the positive and negative electrodes are simply adhered to the solid electrolyte (corresponding to a separator impregnated with an electrolyte) via an adhesive resin, too large resistance of the ion conductive adhesive resin layer causes the ion conductive resistance between the electrodes to increase, thereby deteriorating the battery properties.
In the example disclosed in Japanese Unexamined Patent Publication No. 159802/1993 wherein the positive and negative electrodes are joined with the solid electrolyte via a bonding agent, the interface among the positive and negative electrodes and the solid electrolyte is covered with the bonding agent, so that the battery is inferior to a battery with a liquid electrolyte in terms of conductivity and battery performance due to increased resistance between the electrodes. Even if a bonding agent having ion conductivity is used, it is still difficult to obtain the same level of battery performance as the liquid electrolyte.
The present invention, which has contrived as a result of hard study on the preferable method of adhering to the separator and the positive and negative electrodes to solve the above-mentioned problems, has an object of providing a method for manufacturing a rechargeable lithium ion secondary battery capable of adhering a positive and negative electrodes and a separator without a strong solid casing or without increasing resistance between the electrodes, and of having high energy density, being thinner, and being any desired form.
The first method for manufacturing a lithium ion secondary battery of the present invention comprises the following steps; a step of preparing a positive electrode obtained by joining a positive electrode active material layer with a positive electrode current collector, a negative electrode obtained by joining a negative electrode active material layer with a negative electrode current collector, and a separator arranged between the positive electrode and the negative electrode, a step of supplying a second solvent different from the first solvent to the applied adhesive resin solution after applying an adhesive resin solution, wherein an adhesive resin is dissolved in a first solvent, to the separator, and a step of forming an electrode laminate by laminating the positive electrode and the negative electrode to the separator.
By this method it is possible to join the active material layer with the separator without using a solid casing. In addition, when a porous adhesive resin is formed to electrically connect the active material layer and the separator with the use of an electrolyte, the second solvent different from the first solvent used for the adhesive resin solution applied on the separator is supplied to the applied adhesive resin solution, which realizes a decrease in the fluidity in the vicinity of the surface of the applied adhesive resin solution. As a result, it becomes possible to restrict penetration of the adhesive resin solution into the battery and to improve the adhesion, thereby forming a porous adhesive resin layer having improved bonding strength between the electrodes and the separator via the adhesive resin. And it provides an effect of obtaining a lithium ion secondary battery which has excellent charge-discharge properties and high energy density and can be formed into any desirable shape in a simple manner with good workability.
The second method for manufacturing a lithium ion secondary battery of the present invention is that in the first method the second solvent is a material soluble in the first solvent and having lower solubility of the adhesive resin than the first solvent. In the method, a material, which can be uniformly mixed in a solvent used for the adhesive resin solution but has a lower solubility of the adhesive resin than the first solvent, is used as the second solvent. It has an effect of decreasing fluidity of the adhesive resin solution as compared with the case wherein only the first solvent is used.
The third method for manufacturing a lithium ion secondary battery of the present invention is that in the first method the second solvent is supplied to the surface of the applied adhesive resin solution by spraying droplets of the second solvent through a spray. In the method, spraying the droplets of the second solvent through a sprayer has an effect of decreasing fluidity of the adhesive resin solution in the vicinity of its surface in a simple manner with good workability.
The fourth method for manufacturing a lithium ion secondary battery of the present invention is that in the first method the second solvent is supplied by introducing the separator applied with the adhesive resin solution to space containing vapor of the second solvent. In the method, fluidity of the adhesive resin solution in the vicinity of its surface can be decreased in a simple manner with good workability by exposing the surface of the applied adhesive resin solution to the vapor of the second solvent.
The fifth method for manufacturing a lithium ion secondary battery of the present invention is that in the first method the adhesive resin is either a fluororesin or a mixture comprising a fluororesin as a main component. In the method, by using either a fluororesin or a mixture mainly comprising a fluororesin as an adhesive resin, it becomes possible to obtain a porous adhesive resin layer having a large bonding strength, and consequently, electrode laminate having low resistance between the electrodes and sufficient bonding between the electrodes and the separator can be obtained. As a result, a lithium ion secondary battery having excellent properties can be obtained.
The sixth method for manufacturing a lithium ion secondary battery of the present invention is that in the fifth method the fluororesin is a homopolymer or a copolymer of vinylidene fluoride. In the method, by using the homopolymer or copolymer of vinylidene fluoride as the fluororesin there arises an effect of obtaining an electrode laminate which is electrochemically very stable, has low resistance between the electrodes and sufficient bonding between the electrodes and the separator.
The seventh method for manufacturing a lithium ion secondary battery of the present invention is that in the first method the adhesive resin is either poly(vinyl alcohol) or a mixture comprising poly(vinyl alcohol) as a main component. In the method, by using either poly(vinyl alcohol) or a mixture mainly comprising poly(vinyl alcohol) as the adhesive resin it becomes possible to obtain an electrode laminate which is electrochemically very stable, has low resistance between the electrodes and sufficient bonding between the electrodes and the separator.
The eighth method for manufacturing a lithium ion secondary battery of the present invention is that the first method further comprises a step of forming a structure wherein a plurality of an electrode laminate are piled up. According to the present invention, the sufficient bonding strength and the high ion conductivity are secured, therefore it can provide a structure wherein a plurality of an electrode laminate are piled up, namely a structure of a laminated electrode battery which does not need a strong solid casing. Thus, by forming a structure of an electrode laminate it becomes possible to obtain a compact lithium ion secondary battery having stable battery properties.